This invention relates to the administration of dynamic (i.e. gravity segregating) particulate dispersion systems, e.g. gas-containing diagnostic contrast agents, more particularly to apparatus and a method for the controlled and substantially steady state administration of such dispersions by infusion.
In the field of ultrasonography it is well known that contrast agents comprising dispersions of gas microbubbles are particularly efficient backscatterers of ultrasound by virtue of the low density and ease of compressibility of the microbubbles. Such microbubble dispersions, if appropriately stabilised, may permit highly effective ultrasound visualisation of, for example, the vascular system and tissue microvasculature, often at advantageously low doses of the contrast agent.
Gas-containing contrast media are also known to be effective in magnetic resonance (MR) imaging, e.g. as susceptibility contrast agents which will act to reduce MR signal intensity. Oxygen-containing contrast media also represent potentially useful paramagnetic MR contrast agents.
In the field of X-ray imaging gases such as carbon dioxide may be used as intravascular contrast agents. Moreover, the use of radioactive gases, e.g. radioactive isotopes of inert gases such as xenon, has been proposed in scintigraphy, for example for blood pool imaging.
Gas-containing ultrasound contrast agents are usually administered intravenously as a single or multiple bolus dosage, leading to a rapid and pronounced but relatively short lived rise in backscatter intensity in respect of blood-perfused tissue and organs as the bolus mixes with surrounding blood and is carried through the circulation system. A plot of backscatter intensity against time therefore shows a relatively narrow and high signal intensity peak; backscatter measurements are normally made during the existence of this peak, although this may give rise to problems in, for example, the imaging of deeper tissue and organs where high backscatter from overlying tissue may cause excessive shadowing during the peak period.
As discussed in WO-A-9748337, diagnostic artefacts such as shadowing may be reduced by controlling the rate of administration of the contrast agent and/or by administering a flush such as normal saline after administration of the contrast agent. Contrast agent administration rates of 1-8×106 vesicles/kg-sec or 1×10−7 to 3×10−3 cc gas/kg-sec and flush rates of 0.01-2.4 ml/sec are suggested; the contrast agent is typically administered over a period of 5-20 seconds, and any subsequent flush is typically administered over a period in the range 10 seconds to 10 minutes.
Continuous infusion of ultrasound contrast agents, for example over a period in the range from one minute to one hour, is of potential interest in that it may permit administration of the contrast agent at a rate which minimises diagnostic artefacts such as shadowing and may lengthen the useful time window for imaging beyond the relatively short duration of the backscatter signal peak resulting from passage of a contrast agent bolus.
Thus, for example, Albrecht et al. in Radiology 207, pp. 339-347 (1998) note that the use of continuous contrast agent infusion to provide prolonged enhancement of Doppler signals is advantageous in that it may permit completion of lengthy imaging procedures such as studies of the renal arteries or peripheral leg veins and may optimise dose effectiveness of the contrast agents, as well as reducing saturation artefacts.
Administration of contrast agents by infusion may also be useful in procedures based on imaging of contrast agent in the recirculating phase following admixture with the blood pool, as described in WO-A-9908714.
A problem with the continuous infusion of gas-containing diagnostic contrast agents arises from the tendency of gas-containing components such as microbubbles to float, since this will lead to inhomogeneities forming within vessels such as power-driven syringes which may be used to administer the contrast agent. This may, for example, lead to an increase in microbubble concentration in the upper part of such a vessel and/or to changes in size distribution occurring at various points within the vessel as larger microbubbles float more rapidly than smaller microbubbles.
A possible solution to this problem is proposed in WO-A-9927981, which discloses powered injector systems comprising a syringe which is subjected to rotational or rocking motion in order to maintain homogeneity within the contents thereof. In specific embodiments the barrel of the syringe is positioned horizontally in contact with wheels or moveable brackets which are capable of alternately rotating the syringe in opposite directions about its longitudinal (i.e. horizontal) axis.
It will be appreciated that the incorporation of such rotational or other agitational means into syringe driver apparatus necessarily complicates the design and significantly increases the cost of such apparatus, so that there is an ongoing need for apparatus which permits the continuous infusion of gas-containing ultrasound contrast agents or other gravity segregating dispersions while maintaining substantial homogeneity of the contrast agent or other dispersion.